<!DOCTYPE html>
<html lang="en">

<head>
  <meta charset="UTF-8">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Document</title>
  <style>
    div:hover .box {
      background-color: black;
    }

    div {
      border: 1px solid red;
    }

    .box {
      width: 200px;
      height: 300px;
      background-color: skyblue;
      transition: all 2s linear 1000ms;
    }
  </style>
</head>

<body>
  <div>
    <h1>过渡</h1>
    <div class="box"></div>
  </div>
</body>

</html>